The present invention relates to a method and an arrangement for non-destructive determination of rigidity, tensile and/or structural properties of a preferably oblong and/or plate-shaped object. Determination of the geometrical dimensions of the object are obtained through impact excitation and registration of resonance frequencies of natural modes of the object.
The invention also relates to an assembly including an arrangement according to the invention.
When constructional wood is mechanically strength sorted, classification is generally based on evaluations of the coefficient of elasticity of the wood by applying a static bend load in a pliable direction. This coefficient of elasticity is correlated with the strength of the wood and forms the basis for sorting into strength classes. However, these machines have limited performance and do not have sufficient capacity to characterize high strength wood. The majority of conventional sorting machines require that the wood be transported longitudinally through the machine, while in most cases it would be advantageous, from the production technique point of view, if the machines could manage to perform classification during the continuous cross convey of the wood.
In the laboratory environment, methods based on the measurement of fundamental resonance frequencies at bending and axial vibration, respectively, have been shown to be considerably more accurate than conventional machines for predicting bending strength, as discussed in xe2x80x9cStrength and stiffness prediction of timber using conventional and dynamic methods,xe2x80x9d by Mikael Perstorper, First European Colloquiums on Non-destructive Evaluation of Wood, University of Sopron, Hungary, Sep. 21-23, 1994, vol. 2. The problem with this method is that until now adoption to industrial conditions in respect of speed, automation and continues flow has not been possible.
Until now the fundamental resonance frequency at bending and axial vibration, respectively, have been utilized for predication of strength properties. By using information from multi-channel modes a more reliable characterization of the mechanical characteristics of the measured object is obtained.
SE 348 558 describes a non-destructive method that classifies the wood material by exposing the short end of the sample body for a physical hit to generate an energy wave in the sample body. The wave extends in the longitudinal direction. The time for the passage of the energy wave between two sensors is measured and the sample body is classified depending on its coefficient of elasticity, which is determined by the speed of the energy wave and the density of the sample body.
The prior art is also evident through a number of other patent documents. For example U.S. Pat. No. 4,926,691 teaches a method for measuring rigidity and the condition of a wooden structure, preferably poles dug in the ground. The first five resonance modes are used, which are measured by an accelerometer or velocity transducer. U.S. Pat. No. 4,446,733 shows a system for inducing compressive stress in rigid objects for endurance tests. The sample object is held firmly in a holder at a test moment. U.S. Pat. No. 4,399,701 also shows a method for detecting degradation in wood, preferably wooden poles firmly dug in the ground. According to this document, grooves are arranged in the pole for insertion of acoustic transducers in the pole. Two relatively complicated equipments are known through U.S. Pat. No. 5,207,100 and U.S Pat. No. 5,255,565, which require complicated signal processing. U.S. Pat. No. 2,102,614 describes a method for generating and discrimination of vibrations in an airplane propeller. The propeller is suspended by means of an elastic suspension member and a vibrator is connected to the center of the propeller.
An object of the invention is to provide a method for strength classification of a body, such as wood and other wood-based products in a more accurate, fast and effective way. Another object of the present invention is to provide an industrially applicable technical solution for determination of resonance frequencies of a body for purpose of strength sorting. In a preferred embodiment, the invention can be applied to sample objects, which primarily are continuously and transversely transported.
Experiments have shown that the invention can increase production capacity, for example when classifying wood, about one hundred objects can be classified during one hour compared to the present forty hours.
These tasks have been solved by using a resonance frequency from at least one of the objects natural modes, which resonance frequency is obtained by bringing the object into vibration by means of a stroking body, and essentially controlling the initiation of the movement of the stroking body and following physical impact in time and space through movement of the object. The arrangement according to the invention includes means to bring the object into an essentially free vibration state, a unit for processing collected vibration data and determining rigidity and/or strength of the object alternatively to obtain the geometrical dimension of the object by means of resonance frequencies at least from one of the object""s natural modes.